1. Field of the Invention
The present invention relates to an inkjet image forming method and an inkjet image forming apparatus. Specifically, the present invention relates to an image forming method and an image forming apparatus that form an ink image on an intermediate transfer body by using a inkjet method and transfer the ink image to a printing medium for performing printing.
2. Description of the Related Art
Recently, there is growing a request for taking advantage of an inkjet printing method and outputting an image with high quality using an inkjet printing method regardless of types of a printing medium. For example, there is a request for performing printing onto a printing medium that does not absorb any aqueous ink composition, such as plastics or metal (hereinafter, also referred to as “non-ink-absorbency”). Also, there is a request for performing printing onto a printing medium in which the amount of absorbing aqueous ink compositions is small or the absorption speed for absorbing aqueous ink compositions is slow (hereinafter, also referred to as “low ink absorbency”), such as art paper or coated paper among printing papers. It is desired that when performing printing onto the above-described printing media, for the purpose of improving quality of images, a factor that contributes to reduction in image quality on the printing medium or reduction in texture of printed matters is suppressed. Specifically, phenomena called feathering, beading, and bleeding as well as a waving phenomenon (cockling) of the printing medium caused by permeation of water-based ink into the printing medium are required to be suppressed. Higher precision and speeding up of the inkjet printing method are contributory to some of the above-described phenomena. The phenomena must be more effectively suppressed under the following conditions: a large amount of ink is applied in a high speed per unit area of the printing medium, and further, various materials are being used as the printing media.
To cope with the above-described problem, there is a printing method in which ink is applied to an intermediate transfer body to form an ink image by the inkjet printing method and such ink image is transferred to a printing medium (see Japanese Patent Laid-Open No. H06-182982 (1994), Japanese Patent Laid-Open No. H06-218913 (1994)). This transfer method is such that the ink image is formed on the intermediate transfer body once, the ink image is dried and thereafter the intermediate transfer body is pressed onto the printing medium to transfer the ink image to the printing medium.
According to the printing method using this transfer method, since a phenomenon such as feathering, beading, or bleeding is suppressed, types of applicable printing media can be increased. Further, before formation of the ink image, a processing liquid that causes thickening of ink or aggregation and/or insolubilization of colorant by reacting with the ink is applied to the intermediate transfer body in some cases. As a result, the ink applied to the intermediate transfer body instantly agglomerates to be insolubilized before causing the image degradation such as bleeding, thereby also fixing the ink image with preferable image quality without change.
Further, the use of the intermediate transfer body in the inkjet printing method has the benefit that dusts such as paper powder generated from the printing medium hardly become attached to nozzles. More specifically, since a print head having the nozzles for ejecting ink is disposed at a position distant from the printing medium, clogging caused by the attachment of the paper powder to the nozzles can be suppressed.
Further, the ink image formed on the intermediate transfer body, before the transfer operation to the printing medium, goes through the drying step as a step of removing an extra liquid component contained in the ink image, thereby reducing the liquid volume permeated into the printing medium. Therefore, there is the advantage of hardly causing cockling and that of doing no harm to a texture of the printing medium such as rigidity and touch.
However, in the transfer method, when the level of dryness of the ink image on the intermediate transfer body during the transfer operation is not proper, the ink image cannot be transferred while keeping quality of the image on the intermediate transfer body, and accordingly, the quality of the image formed on the printing medium may be reduced. Specifically, when a drying operation is insufficient, the distortion of the image (hereinafter, also referred to as “image flowing”) or the bleeding readily occurs. On the other hand, when the ink image is overdried, tackiness between the ink image and the printing medium is reduced, and the tackiness between the ink image and the intermediate transfer body surface is relatively strengthened. As a result, a phenomenon in which the ink image is broken into the intermediate transfer body and the printing medium (hereinafter, also referred to as “separation”), and accordingly the ink may remain on the intermediate transfer body also after the transfer operation (hereinafter, also referred to as “transfer residue”). The above-described separation and transfer residue tend to be prominently caused with the progress of drying. Further, also when the drying operation is insufficient depending on types or concentration of the ink, a cohesive force within the ink image is insufficient in a residual solvent, and as a result, the separation may occur.
As described above, in the transfer method, if the transferring the ink image is performed with a dried condition remaining insufficient or over, the degradation of image quality due to drying failure easily occurs. To prevent the degradation of image quality from occurring, the transferring the ink image must be performed in a condition that a residual liquid amount in the ink image on the intermediate transfer body is in an adequate range. FIG. 1 is a diagram showing an adequate range (b≦W≦a) of the residual liquid amount within the ink image on the intermediate transfer body. The vertical axis represents the residual liquid amount W within the ink image and the horizontal axis represents the drying time t. The residual liquid amount W decreases so as to tilt downward in the right with the drying time getting longer. When the residual liquid amount is larger than an upper limit “a” of the adequate range, the image flowing is caused by the shortage of drying. Meanwhile, when the residual liquid amount is smaller than a lower limit “b” of the adequate range, the transfer residue is caused by the over drying. Therefore, the residual liquid amount W during the transfer needs to be within the adequate range. Accordingly, the drying time t must fall within a range of t(a)≦T≦t(b). Here, “t(a)” is a time at which the residual liquid amount W becomes the upper limit “a” and “t(b)” is a time at which the residual liquid amount W becomes the lower limit “b”.
To prevent the degradation of image quality due to the transfer in the insufficient drying or an over drying state from occurring, Japanese Patent Laid-Open No. H07-047760 (1995) discloses a method that repeats a cycle of inkjet image formation, drying and transferring plural time to form an image on one piece of the printing medium.
However, in the method disclosed in Japanese Patent Laid-Open No. H07-47760 (1995), the drying operation of the same powers and the same operation time is performed in all of the plural time of drying operation. As a result, securing stability of the transfer and securing high throughput can not go together.
More specifically, for securing the transfer stability, it is desirable to perform each of the plural time of drying operation with low drying powers and long time (T1). Thus, even if a drying state changes due to change in a surrounding environment (temperature, humidity or the like) of an apparatus (that is, even if the drying state shown by a solid line in FIG. 3A changes into a condition shown by any one of two dotted lines), only performing the drying operation of the predetermined drying time (T1) allows the residual liquid amount at the transfer to be readily within the adequate range. In other wards, even if the drying states changes into the condition shown by any one of the dotted lines, a slope of drying curve designated by the dotted line is small so that a deviated amount by which the residual liquid amount at the time T1 deviates from a predetermined residual liquid amount within the adequate range is small. As a result, the residual liquid amount at the transfer readily has an amount within the adequate range, even if the drying state changes. Accordingly, the transferring an ink image can be performed in the condition in which the residual liquid amount is within the adequate range and therefore transferring failure due to the shortage of drying or the over drying hardly occurs. However, in this case, as apparent from FIG. 3A, total drying time becomes long as T1×3 and thus high throughput can not be realized.
On the other hand, when the drying operation with high powers and shirt drying time as shown in FIG. 3B, high throughput can be realized. However, in this case, if the drying state changes as shown by dotted line in FIG. 3B due to the change in the surrounding environment (temperature, humidity or the like), the residual liquid amount during the transferring is readily outside the adequate range of residual liquid amount though the drying operation is performed for a predetermined drying time (T2). More specifically, contrary to the case shown in FIG. 3A, a deviated amount by which the residual liquid amount at the time T2 deviates from a predetermined residual liquid amount within the adequate range is relatively large. As a result, the residual liquid amount at the transferring readily has an amount outside the adequate range. Accordingly, the transfer failure due to the shortage of drying or over drying readily occurs and thus the transfer stability can not be secured.